This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have been examining neonatal macaques and tissues to determine why SIV/HIV progresses faster in neonates compared to adults. To date, information on T cell turnover rates is limited to peripheral blood, and few studies have examined proliferation and T cell turnover in tissues, particularly the intestinal tract, the primary target for acute SIV and HIV infection. To monitor proliferation of cell subsets in various tissues, we administered bromodeoxyuridine (BrdU) 24 hrs prior to tissue collection. Since BrdU is a thymidine analog incorporated only by cells synthesizing DNA, this method allows for detection of cells in the synthesis phase (S-phase) of cell division. For comparison, cells were also examined for Ki-67 expression, which detects cells at all stages of cell division (G1, S, M, and G2). Proliferation rates of T cell subsets were compared in the blood, lymph nodes, spleen, and intestines of pediatric macaques infected with SIVmac251 and age-matched uninfected controls.